Lab: xv6 lazy page allocation

One of the many neat tricks an O/S can play with page table hardware is lazy allocation of user-space heap memory. Xv6 applications ask the kernel for heap memory using the sbrk() system call. In the kernel we've given you, sbrk() allocates physical memory and maps it into the process's virtual address space. However, there are programs that use sbrk() to ask for large amounts of memory but never use most of it, for example to implement large sparse arrays. To optimize for this case, sophisticated kernels allocate user memory lazily. That is, sbrk() doesn't allocate physical memory, but just remembers which addresses are allocated. When the process first tries to use any given page of memory, the CPU generates a page fault, which the kernel handles by allocating physical memory, zeroing it, and mapping it. You'll add this lazy allocation feature to xv6 in this lab.

Before you start coding, read Chapter 4 of the xv6 book, and related files:

To start the lab, switch to the lazy branch:

  $ git fetch
  $ git checkout lazy
  $ make clean
  

Eliminate allocation from sbrk()

Your first task is to delete page allocation from the sbrk(n) system call implementation, which is the function sys_sbrk() in sysproc.c. The sbrk(n) system call grows the process's memory size by n bytes, and then returns the start of the newly allocated region (i.e., the old size). Your new sbrk(n) should just increment the process's size (myproc()->sz) by n and return the old size. It should not allocate memory -- so you should delete the call to growproc() (but you still need to increase the process's size!).

Try to guess what the result of this modification will be: what will break?

Make this modification, boot xv6, and type echo hi to the shell. You should see something like this:

init: starting sh
$ echo hiusertrap(): unexpected scause 0x000000000000000f pid=3
            sepc=0x0000000000001258 stval=0x0000000000004008
va=0x0000000000004000 pte=0x0000000000000000
panic: uvmunmap: not mapped
The "usertrap(): ..." message is from the user trap handler in trap.c; it has caught an exception that it does not know how to handle. Make sure you understand why this page fault occurs. The "stval=0x0..04008" indicates that the virtual address that caused the page fault is 0x4008.

Lazy allocation

Modify the code in trap.c to respond to a page fault from user space by mapping a newly-allocated page of physical memory at the faulting address, and then returning back to user space to let the process continue executing. You should add your code just before the printf call that produced the "usertrap(): ..." message. Your solution is acceptable if it passes usertests.

A good way to start this lab is by fixing usertrap() in trap.c so that you can run "echo hi" in the shell again. Once that works, you will find some additional problems that have to be solved to make usertests to work correctly. Here are some hints to get going.

If all goes well, your lazy allocation code should result in echo hi working. You should get at least one page fault (and thus lazy allocation) in the shell, and perhaps two.

Usertests

Now you have the basics working, fix your code so that all of usertests passes:

Your solution is acceptable if your kernel passes lazytests and usertests:

$  lazytests
lazytests starting
running test lazy alloc
test lazy alloc: OK
running test lazy unmap...
usertrap(): ...
test lazy unmap: OK
running test out of memory
usertrap(): ...
test out of memory: OK
ALL TESTS PASSED
$ usertests
...
ALL TESTS PASSED
$

Submit the lab

This completes the lab. Make sure you pass all of the make grade tests. If this lab had questions, don't forget to write up your answers to the questions in answers-lab-name.txt. Commit your changes (including adding answers-lab-name.txt) and type make handin in the lab directory to hand in your lab.

Time spent

Create a new file, time.txt, and put in it a single integer, the number of hours you spent on the lab. Don't forget to git add and git commit the file.

Submit

You will turn in your assignments using the
submission website. You need to request once an API key from the submission website before you can turn in any assignments or labs.

After committing your final changes to the lab, type make handin to submit your lab.

$ git commit -am "ready to submit my lab"
[util c2e3c8b] ready to submit my lab
 2 files changed, 18 insertions(+), 2 deletions(-)

$ make handin
tar: Removing leading `/' from member names
Get an API key for yourself by visiting https://6828.scripts.mit.edu/2020/handin.py/
Please enter your API key: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 79258  100   239  100 79019    853   275k --:--:-- --:--:-- --:--:--  276k
$
make handin will store your API key in myapi.key. If you need to change your API key, just remove this file and let make handin generate it again (myapi.key must not include newline characters).

If you run make handin and you have either uncomitted changes or untracked files, you will see output similar to the following:

 M hello.c
?? bar.c
?? foo.pyc
Untracked files will not be handed in.  Continue? [y/N]
Inspect the above lines and make sure all files that your lab solution needs are tracked i.e. not listed in a line that begins with ??. You can cause git to track a new file that you create using git add filename.

If make handin does not work properly, try fixing the problem with the curl or Git commands. Or you can run make tarball. This will make a tar file for you, which you can then upload via our web interface.

Optional challenge exercises